The causes of wide variations observed in medical service utilization across small geographic areas are poorly understood. A better understanding of the role of uncertainty and of physician decision-making is important in evaluating the quality and cost implications of variations in utilization, and formulating appropriate educational and policy responses. Physician uncertainty has been proposed as one cause of such variation, but variations have not been demonstrated to result from differences in doctors' decisions (as opposed to sociodemographic differences or differences in demand for or access to care) . Nor have physicians been demonstrated to vary across areas in their responses to patients presenting uncertainty. This proposal is for a study comparing doctors' hospital admission decision-making under conditions of uncertainty between two demographically similar communities with differing rates of hospital utilization. The Michigan Inpatient Data Base will be used to perform a small-areas utilization analysis for the State of Michigan for the spectrum of diagnoses comprising actual or suspected acute ischemic heart disease. Decisions to admit or release approximately 1000 patients presenting with acute ischemic heart disease to the emergency departments of hospitals in the two communities will be retrospectively examined. The Acute Ischemic Heart Disease Predictive Instrument will be applied to each medical record to determine a probability of ischemic disease, and patients with low but nonzero probabilities will be considered to present diagnostic uncertainty. The primary aims of this study are to demonstrate that differences in hospital utilization rates for acute ischemic heart disease in the two communities it directly from different physician decision-making patterns, and to show that the differences in decision-making are concentrated among those patients presenting the greatest uncertainty in diagnosis.